This invention relates to the preparation of latexes made from starches derived from annually renewing agricultural sources, and vinyl monomers derived from annually diminishing fossilized sources. The products of our invention are as effective in a number of applications as conventional latexes made wholly from vinyl monomers. Therefore, the practice of our invention can reduce cost and conserve petrochemicals because less of the vinyl component which is more costly than starch is required in our novel latexes.
Review articles (Graft Copolymers, H. A. Battaerd and G. W. Tregear, eds., Interscience Publishers, 1967; and Block and Graft Copolymers, R. J. Ceresa, ed., John Wiley and Sons, 1973) cover the preparation and development of starch graft polymers prior to 1973. These articles and the literature thereafter up to the work of Gugliemelli et al. (J. Polym. Sci. 12: 2683 (1974)) do not mention starch graft latex products or concepts.
It is known that latex preparation is dependent on particle size and particle stability. Particle sizes should be on the order of a micron or less.
Stability of latexes prepared by classical emulsion polymerization (Principles of Polymer Chemistry, P. J. Flory, Univ. Press, Ithaca, New York, 1953) is attributed to polymerization of vinyl monomers within soap or detergent micelles to form polymer particles that are stabilized subsequent protective colloid action of adsorbed surfactants. Merely grafting vinyl monomers onto granular starch or cationic starches produce insoluble products whose aqueous slurries dry to opaque powders of no demonstrable utility.
We were surprised, therefore, when we discovered that stable latexes could be formed without the use of soaps, detergents, or emulsion polymerizations by the following steps:
A. graft polymerizing gelatinized cationic starch with a vinyl monomer in quantities such that a vinyl monomer add-on of from about 10% to 200% occurs in the presence of cerium(IV) initiator in an aqueous media containing from 1% to 20% solids; and PA1 B. sonicating the dispersion resulting from step (a) at 20 KHz for 1-3 minutes to form a stable dispersion containing cationic starch graft latex solids having particle diameters of from 300 to 1500 A, said stable dispersion having a Brookfield viscosity of from 10 to 100 cp.
From our disclosure of reaction and reactant parameters in making starch graft latexes it will be evident to those skilled in the art how to prepare a great variety of latexes consisting of micro-particles which in turn are made of gelatinized starch or gelatinized cationic starch and grafted vinyl polymer side chains. some polyvinyl side chains may have relatively high glass transition temperatures (250.degree.C.) which permit little polymer-flow and others of low glass transition temperatures (0.degree. C.) which permit much polymer-flow.
Starch graft copolymer latexes were found to have utility in a number of areas of application depending on polyvinyl moieties and compositions. Starch latexes having small particle sizes and polyvinyl moieties of high glass transition temperatures as in the case of starch-polyacrylonitrile were found to function well as reinforcers in rubber. Those latexes having strong cationic character functioned well as additives in wet-end addition in paper making by imparting dry strength to paper. In particular, latexes having polyvinyl unsaturation as in the case of starch-polychloroprenes were useful in imparting wet strength in paper. Also starch graft latexes made from dienes or methylacrylates were useful as cold-contact adhesives in glueing wood. Starch graft latexes dry at 25.degree. C. or at higher temperatures to form clear adhesive films.